module: do not warn for typeless package.json when there isn't one

It was intended that warnings should only be emitted for an
existing package.json without a type. This fixes a confusing
warning telling users to update /package.json when there are
no package.json on the lookup path at all, like this:

[MODULE_TYPELESS_PACKAGE_JSON] Warning: ... parsed as an ES module
because module syntax was detected; to avoid the performance penalty
of syntax detection, add "type": "module" to /package.json

Drive-by: update the warning message to be clear about
reparsing and make it clear what's actionable.

Author: joyeecheung

lib/internal/modules/esm/get_format.js

@@ -98,8 +98,9 @@ let typelessPackageJsonFilesWarnedAbout;
 function warnTypelessPackageJsonFile(pjsonPath, url) {
   typelessPackageJsonFilesWarnedAbout ??= new SafeSet();
   if (!typelessPackageJsonFilesWarnedAbout.has(pjsonPath)) {
-    const warning = `${url} parsed as an ES module because module syntax was detected;` +
-      ` to avoid the performance penalty of syntax detection, add "type": "module" to ${pjsonPath}`;
+    const warning = `Module type of ${url} is not specified and it doesn't parse as CommonJS.\n`+
+      `Reparsing as ES module because module syntax was detected. This incurs a performance overhead.\n` +
+      `To eliminate this warning, add "type": "module" to ${pjsonPath}.`;
     process.emitWarning(warning, {
       code: 'MODULE_TYPELESS_PACKAGE_JSON',
     });
@@ -118,7 +119,7 @@ function getFileProtocolModuleFormat(url, context = { __proto__: null }, ignoreE
   const ext = extname(url);
 
   if (ext === '.js') {
-    const { type: packageType, pjsonPath } = getPackageScopeConfig(url);
+    const { type: packageType, pjsonPath, exists: foundPackageJson } = getPackageScopeConfig(url);
     if (packageType !== 'none') {
       return packageType;
     }
@@ -139,7 +140,7 @@ function getFileProtocolModuleFormat(url, context = { __proto__: null }, ignoreE
         // For ambiguous files (no type field, .js extension) we return
         // undefined from `resolve` and re-run the check in `load`.
         const format = detectModuleFormat(source, url);
-        if (format === 'module') {
+        if (format === 'module' && foundPackageJson) {
           // This module has a .js extension, a package.json with no `type` field, and ESM syntax.
           // Warn about the missing `type` field so that the user can avoid the performance penalty of detection.
           warnTypelessPackageJsonFile(pjsonPath, url);
@@ -170,7 +171,7 @@ function getFileProtocolModuleFormat(url, context = { __proto__: null }, ignoreE
         const parsedSource = tsParse(source);
         const detectedFormat = detectModuleFormat(parsedSource, url);
         const format = detectedFormat ? `${detectedFormat}-typescript` : 'commonjs-typescript';
-        if (format === 'module-typescript') {
+        if (format === 'module-typescript' && foundPackageJson) {
           // This module has a .js extension, a package.json with no `type` field, and ESM syntax.
           // Warn about the missing `type` field so that the user can avoid the performance penalty of detection.
           warnTypelessPackageJsonFile(pjsonPath, url);

test/es-module/test-esm-detect-ambiguous.mjs

@@ -352,6 +352,18 @@ describe('Module syntax detection', { concurrency: !process.env.TEST_PARALLEL },
       });
     }
 
+    it('does not warn when there are no package.json', async () => {
+      const { stdout, stderr, code, signal } = await spawnPromisified(process.execPath, [
+        fixtures.path('es-modules/loose.js'),
+      ]);
+
+      strictEqual(stderr, '');
+      strictEqual(stdout, 'executed\n');
+      strictEqual(code, 0);
+      strictEqual(signal, null);
+    });
+
+
     it('warns only once for a package.json that affects multiple files', async () => {
       const { stdout, stderr, code, signal } = await spawnPromisified(process.execPath, [
         fixtures.path('es-modules/package-without-type/detected-as-esm.js'),